The present off-the-shelf seismometer consists of a sophisticated, expensive (and usually delicate) spring mass system together with a moving coil-magnet transducer. For seismometers with resonant frequencies of 0.5 Hz and above, these mechanical suspensions are rectilinear, consisting of large springs and stiffeners to prevent lateral movement. For seismometers with lower resonant frequencies (for recording longer-period waves) a pendulous spring-mass system is used. Resonant frequencies as low as 0.01 Hz can be achieved. As a class, these pendulous instruments suffer from a suite of mechanical problems: non-linear, parametric, and exaggerated tilt response; mechanical drift; "spring ringing" excited by high-frequency waves; and general lack of robustness. As mentioned previously, both the modern pendulous and rectilinear seismometers utilize coil-magnet combinations to transduce the mass motion to output voltage. This type of transducer also serves the useful function of removing the D.C. and low frequency variations of mass position. These coils can be made with large generator constants ##EQU1## by using many thousands of turns and high magnetic fields (Sm-Co based magnets produce 8-10 K-Gauss). The coil usually operates directly into an operational amplifier. Since the input voltage noise (output voltage noise referred to the input) of a selected op-amp is as low as 0.1.mu. volts rms over the range 0.1-10 Hz, the threshold velocity sensitivity of this class of modern seismometers is given by: ##EQU2## This corresponds to 0.02 nm rms displacement at 1 Hz or 6.times.10.sup.-8 gals or 6.times.10.sup.-11 g's rms. This figure is somewhat optimistic because the op-amp input noise current has been neglected. However, as described elsewhere, this is well below the seismic background level for almost all surface sites. The practical result is that conventional seismometers of this type are deployed worldwide by the thousands.
As it became desirable to record waves of longer periods such as Rayleigh waves, seismometers were developed with increasingly low resonant frequencies. Because of their complexity, the mechanical system suffers increasingly from problems of drift, mechanical instability, and non-linear response. To avoid many of these difficulties, electronic feedback was added to stabilize the mechanical spring-mass system. Since the transfer function of the system then depends mainly on the feedback, the resonant frequency of the spring-mass system need not be so low.
The earth's gravitation field is approximately 10.sup.6 mgal (mgal=cm/sec.sup.2). Variations of 0.3 mgal are produced by the gravitational influence of the moon and sun. The maximum value of many of the anomalies of interest are 1 mgal or less and values of 0.05 mgal or better must be detectable. The various instruments which have been used to measure gravitational fields are pendulums, torsion balances, and gravimeters. The first two are obsolete. Gravimeters are of two basic types: stable and unstable. In the case of stable gravimeters, extremely small displacements due to gravity must be greatly magnified by optical electrical or mechanical means. An example of a stable type is a flat spring wound in a helix with the flat surface parallel to the axis. A mass attached to the spring will cause the spring to rotate as the result of gravitational changes. The rotational motion is much greater than the displacement. Most current gravimeters are the unstable type where changes from equilibrium are counteracted by other forces which decrease the displacement caused by gravity alone An example of this type is the Lacoste and Romberg gravimeter. Springs are often used in gravimeters. These require the thermostatic regulation. A temperature change of 0.002.degree. C. is equivalent to a gravity change of 0.02 mgal in the Lacoste and Romberg gravimeter. Spring devices suffer from drift due to slow creep in the spring. Drifts of 0.01 to 0.02 mgal per hour are not unusual. In addition, temperature regulation is usually required in order to measure small effects